A wide variety of electronic timer circuits are known in the art, including both analog timer circuits and digital timer circuits. Conventional analog timer circuits based on resistor-capacitor (RC) time constants are advantageous in that such circuits can be configured to provide a substantially continuous analog output and thus a large number of possible set points. However, such analog timer circuits can be problematic in applications in which relatively long time constants, on the order of tens, hundreds or thousands of seconds, are needed, as large and costly capacitors can be required.
Digital timer circuits based on digital counters overcome some of the disadvantages of analog timer circuits. For example, digital timer circuits can provide relatively long time constants. However, such digital timer circuits exhibit only a limited number of set points, corresponding to respective predetermined discrete output times, as compared to the much larger number of set points provided by the continuous analog output of an analog timer circuit. Moreover, digital timer circuits are generally more complex than analog timer circuits, and some of the digital circuit elements can be very costly.
Various techniques are also known for increasing the effective capacitance value of a given capacitive element. For example, the well-known Miller effect provides an increase in the effective capacitance of a given capacitor when the capacitor is used as a negative feedback element in a grounded-emitter bipolar transistor amplifier. In this configuration, the increase in effective capacitance approaches the current gain or beta of the transistor. A similar increase in effective capacitance can be achieved in an emitter-follower circuit in which the capacitor is connected between the base and collector of the bipolar transistor. As the maximum beta value for a bipolar transistor is typically about 100, the increase in effective capacitance in these and other similar conventional arrangements is at most about 100:1, which is generally insufficient to overcome the above-noted issues arising in conventional analog timer circuits based on RC time constants.